hostd & vpxa & vpxd

One of my students, asks me about the difference between vpxa & hostd.

hostd (daemon) is responsible for the performing main management task of the ESXi, like virtual machine operations (such as Power-On, Migration & ...)

But what is going on when we join the ESXi to the vCenter server? 

Now it's time fo the vpxa (agent) to come. vpxa is the related agent for communication between ESXi and vCenter Server, so whenever you try to add the host to the vCenter, vpxa will be started automatically.

Despite the hostd is used for managing most of ESXi operations, the vCenter calls the vpxa to send its commands to the hostd agent. Actually, vpxa is like an intermediate between the hostd of the ESXi and vpxd (daemon) on the vCenter to pass the executed commands from the vCenter server to the ESXi (TCP/UDP Port 902). Although if you want to manage the ESXi directly, management communication will handle by the host itself (UDP Port 902).

vpxd (daemon) also acts as a part of the vCenter server and is the responsible for sending commands via vpxa agent to the ESXi hostd service. (Also if the vpxd is stopped, you cannot connect to the vCenter server via the vsphere client.)

vCenter Server --> vpxd --> vpxa ---> hostd --> ESXi

For restarting the ESXi host daemon and vCenter Agent services, you can run the following commands:

/etc/init.d/hostd restart

/etc/init.d/vpxa restart